飞秒激光光镊轴向力的计算与分析

Computing of the Axial Optical Force Applied by the Femtosecond Laser Trap

光镊已成为捕获和操纵微米尺度粒子和生物细胞的有效手段 ,而目前常用的光镊光源为连续激光或长脉宽的脉冲光。提出飞秒激光光镊的概念 ,将飞秒激光序列脉冲视为对连续光的周期抽样 ,借助于连续光光镊的分析方法 ,建立了飞秒激光光镊对电介质微粒产生的轴向光学力的理论模型。给出影响捕获微粒的主要因素 ,指出存在最佳束腰半径和被捕获粒子半径。数值计算结果表明选取合适的飞秒激光脉冲能量、束腰半径、脉冲波长以及微粒与周围媒质的相对折射率 ,微米尺寸的微粒完全能被飞秒激光稳定捕获。综合考虑被捕获微粒所受的脉冲式光学梯度力、重力和布朗惯性力的作用 。

Optical tweezers are one of the most efficient means of manipulating objects on the micrometer scale and biological cells. The sources of the existing optical tweezers are CW light or long pulse width light. The concept of femtosecond laser tweezers was proposed. Taking femtosecond pulses as the sampling of CW lasers, the theoretical model of the induced axial forces by femtosecond laser pulses exerted on micrometer sized sphere was established. The major factors that affect the trapping were discussed and suitable beam waists and radii of the trapped particles were demonstrated. The numerical results show that micron sized particles can be trapped with femtosecond pulses with suitable single pulse energy, beam waist, pulse wavelength and the relative refraction index of the particle to the medium. Considering the axial optical force, the gravity and the Brownian inertial force, the pulsed property of the axial gradient force and the stability condition of femtosecond laser tweezers were also demonstrated.